A *long* while back I asked a question here that was never answered.
The scenario/question was.........
Connected to the ISP's modem is a home rtr. In this case, a wired-only Linksys of which the m/n escapes me now. The Linksys box is NAT'g for the internal network of192.168.1.0/24.
Also attached to the network is a 'real' rtr, with 2 eth interfaces. One of the NIC's is on the192.168.1.0/24 subnet, the other NIC is on the 192.168.2.0/24 subnet connected to a different switch with a couple PC's attached as well. The 'real' rtr is is configured to a bare minimum, only the IP info of each interface set and G/W & DNS are pointing to the ISP rtr, 192.168.1.1.
The question was........will the inexpensive consumer rtr properly NAT and pass the 'off-subnet' routed packets ?
The answer is yes, it will. (This one anyway.) But, one of the requirements is that a route needs to be placed in the ISP rtr for the 192.168.2.0/24 gateway, being the 192.168.1.x address of the 'real' rtr.
My theory is that if the home rtr that is being used has a section in the setup for entering static routes, the above scenario will most likely work. To be clear, these entries are not for port mapping in the ISP rtr, and are usually entitled 'Routes' or 'Routing' in (one of) the 'Advanced' sections.
Just a worthless tidbit, ignore it if you like.
(The history behind the question was that I was thinking about sharing my internet connection with a relative, but wasn't real keen on the Layer2 bridging way of doing it and having all my LAN traffic up in the air. Plus, I already have several complete PtP pairs of some (of The Old)Cleawire proprietary system that operates on Layer3, which would have put the other end on a different subnet.)